1. Field of the Invention
This invention generally relates to a piezoelectric acceleration sensor used for detecting a shock and acceleration applied to an object, and more particularly, to an acceleration sensor for detecting a characteristic amount generated by an inertial force, which is caused resulting from acceleration.
2. Description of the Related Art
In recent years, memory density of a hard disk drive (hereinafter simply referred to as HDD) has been becoming higher at a rapid pace. For this high memory density, a precise control of head positioning is one of the important factors in performing read/write.
For instance, in the case where a chassis of HDD also makes a rotational vibration when affected by another electronics device such as a DVD, CD-ROM, or the like mounted on a personal computer (hereinafter simply referred to as PC), the rotational vibration should be detected and stopped to keep controlling the head positioning.
The acceleration sensor is generally used for the detection of the rotational vibration. Specifically, two acceleration sensors are used for detecting the rotational vibration according to differences in outputs thereof.
Also, portable electronics devices, which have widely been used in recent years, are required to detect and stop the shock in order to maintain the reliability even in the case where an unexpected shock is applied thereto. The above-mentioned acceleration sensor is widely employed in the portable electronics devices such as a notebook-size PC, desktop PC, or the like, in order to prevent an error in the read-in/read-out in the case where an unexpected shock is applied to a HDD which is incorporated into the above-mentioned portable electronics device.
With the advancements of the electronics devices into which the acceleration sensors are incorporated, it is necessary to lower the noises of these electronics devices. An amplifier for amplifying the electrical charge has been proposed to lower the noises. The amplifier for amplifying the electrical charge is capable of lowering line noises and is provided on a signal amplification circuit. Therefore, a high sensitivity to electrical charge is required for the acceleration sensor to improve matching capabilities with an electrical charge amplification circuit.
FIG. 1 shows a conventional piezoelectric acceleration sensor 100. Referring to FIG. 1A, the acceleration sensor 100 includes a piezoelectric vibrator 102 and a circuit board 103. The piezoelectric vibrator 102 is provided on the circuit board 103. A weight 101 is provided on the piezoelectric vibrator 102. The piezoelectric vibrator 102 is polarized in Z direction in FIG. 1A. The weight 101 has a center of gravity outside the piezoelectric vibrator 102.
The piezoelectric vibrator 102, referring to FIG. 1B, includes a capacitance Cd1 composed of electrodes 103a and 103b. FIG. 1C shows an equivalent circuit of the piezoelectric vibrator 102 shown in FIG. 1B. With this configuration, when a vibration or shock is applied to the acceleration sensor 100 (in particular, the weight 101), an electric charge generates on the piezoelectric vibrator 102 (a generated electric charge Q1)
Thus, in a conventional technique 1, the generated electric charge Q1 on the piezoelectric vibrator 102 of the acceleration sensor 100 is detected as a voltage value or charge value, based on a voltage sensitivity SV (See an expression 1) dependent on the capacitance Cd1 or a charge. sensitivity SQ (See an expression 2). It is thus detected that the vibration or shock has been applied.SV=Q1/Cd1  (expression 1)SQ=SV*Cd1  (expression 2)
As an improved technique of the charge sensitivity of an acceleration sensor, Japanese Patent Application Publication No. 2000-121661 discloses a charge-sensitive acceleration sensor 200. FIG. 2 shows the acceleration sensor 200 as a conventional technique 2.
Referring to FIG. 2A, the acceleration sensor 200 includes a piezoelectric vibrator 202 having a double layer structure. FIG. 2B shows an equivalent circuit of the piezoelectric vibrator 202. With this configuration, sensing electrodes of a piezoelectric material are connected in parallel to adjust the polarized directions. The capacitance becomes larger and thus improves the detection sensitivity to the charge.
However, with the acceleration sensor that employs the layered piezoelectric material as described in the conventional technique 2, it is required that the piezoelectric materials are multilayered, polarized axis of each piezoelectric material is adjusted, and the sensing electrodes are intricately connected. This results in problems, complicated configuration and complex manufacturing. These problems lead to another problem in cost reduction. In addition, with the configuration of the conventional technique 2, the voltage sensitivity is lowered. This causes further another problem in that a detection signal is hard to be separated from the noises.
The present invention has been made in view of the above-mentioned circumstances and provides an acceleration sensor with a simple configuration, while maintaining the voltage sensitivity and improving the charge sensitivity.